Mineral oil base lubricants show a significant
decrease of the kinematic viscosity with rising
temperature, as exemplified in Fig. 1 by lubricants
for vehicle gears. An important attribute of lubricants
is their viscosity index (VI) acc. DINAS0 2909 [5].
This is a calculated coefficient, which characterizes
the change of viscosity of lubricants as a function of
temperature. A high viscosity index represents a low
variation of viscosity due to temperature and vice
versa. A low viscosity-temperature-dependence is
required for lubricants, which are operated at significantly
varying temperature conditions, such as
vehicle engine and gear lubricants in summer and
winter time. This way, the oils remain flowing and
pumpable at low temperatures on the one hand, and
on the other hand sufficiently thick lubricant films can be formed at higher temperatures for a safe
separation of the surfaces.
A deliberate improvement of the viscosity-temperature-
behaviour can be achieved by blending an oil
with polymer additives as viscosity-index-improvers,
such as polyalkylmethacrylate (PMA), olefincopolymer
(OCP), styrene-butadiene-copolymer
(SBC) or polyisobutylene (PIB). This way, polymerfree
monograde oils can be converted into multigrade
oils, which are used in large quantities as engine
oils in vehicles (e.g. SAE 15W-40), as gear oils in
manual transmissions in vehicles (e.g. SAE 75W-90,
see Fig. 1) or in automatic transmissions (e.g. ATF
DEXRONB type oils). Universal oils for tractors and
earth-movers (e.g. Urro), which can be used for
hydraulics and gears, are multigrade oils as well.
The viscosity-increasing effect of polymer
additives at laboratory conditions is known and can
be proved. However, their efficiency at EHL-conditions
of high pressure, high shear rate and high
temperature, such as in gear or roller bearing
contacts, is questionable. Investigations by other
authors, e.g. Spikes [12, 15, 161, show that the film
thicknesses formed by polymer-containing oils are
usually less than those predicted from EHL-theory.
Spikes's investigations are based on optical film
thickness measurements, where a steel ball is
pressed against a glass disk with a maximum
Hertzian contact pressure of 520 N/mm2 in the point
contact. The film thickness measurements described
in this paper, however, were carried out using a
capacitance test method with two steel disks
pressed against each other. The maximum Hertzian
contact pressure in this line contact is about 1,200
N/mm2, which is significantly closer to the loads in
actual gear applications.
The lubricant film thickness significantly influences
the micro pitting and wear performance of
gear applications and to a smaller extent it also
affects the pitting and scuffing performance.
Knowledge of the actual lubricant film thickness in a
gear contact is therefore essential for a reliable
estimation of the failure risks of wear and micro
pitting, which are recently more and more in the
focus of interest.
For this reason, the efficiency of VI-improvers in
EHL-contacts was investigated by systematic
measurements of the lubricant film thickness of
more than 20 well defined polymer-containing oils.
Various types of polymers with various molecular weights and concentrations in the base oil were
included in the test program in order to determine
the influence of each parameter on the formation of
lubricant films separately from each other.